A letter was read out in 2,500 parishes in England and Wales during Sunday services, asking Catholics to ensure the true meaning of marriage is not lost for future generations.

The letter from Archbishop Vincent Nichols, the leader of the Catholic Church in England and Wales, and Archbishop Peter Smith, the Archbishop of Southwark, said it was their "duty" to defend the institution of marriage.

"Changing the legal definition of marriage would be a profoundly radical step. Its consequences should be taken seriously now," Archbishop Nichols and Archbishop Smith said in the letter.

"We have a duty to married people today, and to those who come after us, to do all we can to ensure that the true meaning of marriage is not lost for future generations."

This week the British government will publish a consultation paper on changing the definition of marriage to cover same-sex couples.

Prime minister David Cameron says his government is about equality.

One week ago the most senior Catholic cleric in Britain, Cardinal Keith O'Brien, the leader of the Catholic Church in Scotland, said the plans were "madness".

But a poll in the right-leaning Sunday Telegraph newspaper showed public support for the proposals, with 45 per cent saying they supported the move to legalise gay marriage, 36 per cent opposing it.

Civil partnerships for same-sex couples were introduced in Britain in December 2005, giving them similar rights to married heterosexual couples.